The history of modern political journalism begins with Bill Clinton's underpants. This was not, as you may assume, anything to do with Monica Lewinsky, but resulted from an encounter - entirely non-physical - with a schoolgirl. During a high-school Q&A, she asked the president if he was a boxers or a briefs guy. Clinton chose to answer, endorsing the roomier garment, and so gave presidential blessing to a shift in priorities. A politician's policy on knickers was now at least as important as his position on Nicaragua.

During the Tory leadership contest, David Cameron and David Davis were asked on Woman's Hour if they dressed with their testicles supported or free-floating. Even Britain's leading political interrogator was outed under the trousers: Jeremy Paxman received far more newspaper coverage than for any interview he had conducted, over his reported complaints about the gussets in Marks & Spencer underwear.

Gordon Brown and Barack Obama have so far thankfully kept silent on the subject. Both, though, have this week been victims of post-pants reporting. While the prime minister tried to project big policies at the G8 summit, the headlines were hijacked by an unwise remark in an interview with the New Statesman in which he seemed to accept comparison with Emily Brontë's Heathcliff.

And Obama has been wobbled by a brief interview that he, his wife and children gave to the frothy showbiz network, Access Hollywood. The content is anodyne - the senator's daughters speak mainly of ice-cream - but it has undermined Obama's appeals for the privacy of his family to be respected.

Revealingly, in a statement of regret, the Democratic candidate expressed surprise at the speed with which news of his girls' taste in desserts had spread. And it would be safe to guess that our prime minister's volcanic level of irritation with his coverage has been topped up by the fact that an off-the-cuff comment on Wuthering Heights makes the front pages while policy speeches earn a deeply buried paragraph.

Brown and Obama probably blame a media retreat from seriousness, but politicians bear at least half the responsibility for the rise of boxer-shorts reporting. A favoured tactic of political chiefs of staff in the past two decades has been the "soft" interview, in which the leader reveals a lighter side. So, in the 1997 election, it was Ulrika Jonsson who got the big TV encounter with John Major, while Tony Blair, for large stretches of his premiership, was more likely to be seen on Richard & Judy than Newsnight.

Initially this strategy worked, allowing politicians to project normality and casualness - but the media understandably became resentful at being prevented from proper scrutiny. George Bush has relied almost entirely on "soft" appearances in the eight years since he ran for the White House, as did Boris Johnson when running for London mayor. Journalists retaliated by giving their fluffy mutterings the space once reserved for serious speeches.

It's true that this shift was not purely a chess move in a game of spin, but also part of a general drift in the media. Material classified as difficult increasingly required a populist twist to win admission to the news lists.

An interview with a Nobel prize-winning writer, for example, will now receive widespread coverage only if the laureate happens to mock the Harry Potter novels. A scientist who has had the nod from Stockholm will attract international attention only if his thesis can be reduced to a big simplification: that a certain food causes, or cures, cancer; or that there are, or are not, men on Mars. Similarly with politics, editors were often happier to be reporting what politicians had in the sock-drawer rather than the in-tray.

But the result was that supposedly easy questions became dangerous because of the scrutiny they received. Clinton's unwise answer drew attention to the part of his anatomy that would subsequently almost end his presidency. Nick Clegg's career will always be overshadowed by submitting to a bloke mag's questionnaire on his sex life. Malia and Sasha Obama's remarks on ice-cream may be the tool that media lawyers use to invade their privacy at a future point.

Brown's New Statesman chat was another such miscalculation. The line of questioning - are you romantic ... what will you buy the missus for her birthday? - was clearly calculated to humanise him. But apparently gentle questions can be treacherous: while Brown could have quoted whole books on economic theory, he didn't know Wuthering Heights well enough to spot the trap of pairing himself with Heathcliff.

This confirmation that soft questions can give politicians a hard time should make spin doctors realise their favoured tactic is backfiring. And perhaps the benefit of this - for both sides - will be a return to the virtues of hard news.